The present invention relates generally to methods, apparatus and computer program products for determining the vertical speed of an aircraft and, more particularly, to methods, apparatus and computer program products for determining the vertical speed of an aircraft for use in a ground proximity warning system.
Within aviation, the vertical speed or vertical velocity of an aircraft is an important flight parameter and is utilized in a variety of different manners. For example, a traffic collision avoidance system (TCAS) utilizes the vertical speed of an aircraft in its determination of aircraft separation and the performance of other navigation maintenance management functions.
A measure of the vertical speed of an aircraft can be provided by one of several different types of avionics equipment conventionally carried by commercial aircraft. For example, an inertial navigation system (INS) or an initial reference system (IRS) can provide signals indicative of the vertical speed, as well as the acceleration, attitude, altitude, position, magnetic heading/track, true heading/track and ground speed of an aircraft. Alternatively, an air data computer (ADC) can provide signals indicative of vertical speed, as well as the altitude, the computed airspeed, the corrected altitude, the true, airspeed and the static air temperature.
By way of example, one particularly significant avionics subsystem that utilizes the vertical speed of the aircraft, as well as a number of other flight parameters, is a ground proximity warning system. Ground proximity warning systems, also known as terrain awareness systems, analyze the flight parameters of the aircraft, including the vertical speed, and the terrain surrounding the aircraft. Based on this analysis, these warning systems provide alerts to the flight crew concerning possible inadvertent collisions of the aircraft with surrounding terrain or other obstacles, including instances in which the flight path of the aircraft would appear to bring the aircraft in short of the runway.
Ground proximity warning systems often have several modes in order to provide various types of alerts depending upon the flight conditions. For example, the enhanced ground proximity warning system provided by Honeywell, Inc. has six primary modes of operation, at least two of which are dependent upon the vertical speed of the aircraft. In this regard, Mode 1 is designed to provide alerts for an aircraft having an excessive descent rate, i.e., a negative vertical velocity with an excessively large magnitude, that is relatively close to the underlying terrain. Mode 2 provides an alert in instances in which an aircraft is closing with the terrain at an excessive rate, even in instances in which the aircraft is not descending. Mode 3 provides alerts in instances in which an aircraft loses significant altitude immediately after take off or during a missed approach. Mode 3 is activated and deactivated, however, based upon the vertical velocity of the aircraft. Mode 4 provides alerts for insufficient terrain clearance based upon the phase of flight and the speed of the aircraft. In this regard, Mode 4 provides alerts based upon different criteria depending upon whether the aircraft is in the take off phase of flight or in the cruise or approach phases of flight and further depending upon whether the gear is in a landing configuration. Mode 5 also provides two levels of alerts when the aircraft flight path descends below the glideslope beam on front course instrument landing system (ILS) approaches. Finally, Mode 6 provides alerts or call-outs for descent below predefined altitudes or the like during an approach, as well as alerts for excessive roll or bank angles.
In addition to the various modes of operation, the enhanced ground proximity warning system provided by Honeywell, Inc. defines an alert envelope and, more particularly, both a caution envelope and a warning envelope. The imaginary alert envelopes move with the aircraft and are constructed to extend forwardly of the aircraft and to define a region in which alerts will be generated if terrain or other obstacles enter by penetrating one of the alert envelopes. In this regard, U.S. Pat. No. 5,839,080 to Hans R. Muller et al. and assigned to AlliedSignal Inc. describes an advantageous ground proximity warning system that generates an alert envelope. The contents of U.S. Pat. No. 5,839,080 are hereby incorporated by reference in their entirety.
As described by U.S. Pat. No. 5,839,080, an alert envelope is defined by a number of parameters, including a look ahead distance (LAD), a base width (DOFF) and a terrain floor (xcex94H). In general terms, the look ahead distance defines the distance in advance to the aircraft that the alert envelope extends. Similarly, the base width is the lateral width of the alert envelope at a location proximate the aircraft. Further, the terrain floor typically defines a vertical distance below the aircraft that is utilized during the construction of the floor of the alert envelope. Oftentimes, the terrain floor slopes downwardly or upwardly depending upon the flight path angle of the aircraft which, in turn, is at least partially dependent upon the vertical speed of the aircraft. Accordingly, the construction of the alert envelope is partly dependent upon the vertical speed of the aircraft.
As described by U.S. Pat. No. 5,839,080, the ground proximity warning system can construct a pair of alert envelopes, namely, a caution envelope and a warning envelope, that are each partly dependent upon the vertical speed of the aircraft as described above. While each envelope has a similar shape as described above, the caution envelope typically extends further ahead of the aircraft than the warning envelope and is therefore generally larger than the warning envelope. Accordingly, the ground proximity warning system will generate cautionary alerts in instances in which the upcoming terrain or other obstacles penetrate the caution envelope, but not the warning envelope. Once the upcoming terrain or other obstacles penetrate the warning envelope, however, the ground proximity warning system will generate a more severe warning alert. As such, a pilot can discern the severity of the alert and the speed with which evasive maneuvers must be taken in order to avoid the upcoming terrain or other obstacles based upon the type of alert that is provided, i.e., a less severe cautionary alert or a more severe warning alert.
While ground proximity warning systems have substantially improved the situational awareness of flight crews of commercial aircraft by providing a variety of alerts of upcoming situations that merit the attention of the flight crews and by providing graphical displays of the upcoming terrain, obstacles and other notable features, ground proximity warning systems generally require a relatively robust set of input parameters, including the vertical speed of the aircraft as noted above. For example, conventional ground proximity warning systems require a signal indicative of the radio altitude from a radio altimeter, signals indicative of the altitude, the computed airspeed, the corrected altitude, the barometric altitude rate, i.e., the vertical speed, the true airspeed and the static air temperature from an Air Data Computer (ADC), signals indicative of the position, the magnetic track and the corrected altitude from a Flight Management System (FMS), signals indicative of the acceleration, attitude, altitude, vertical speed, position, magnetic heading/track, true heading/track and ground speed from an inertial reference system (IRS), an inertial navigation system (INS) and/or an attitude heading reference system (AHRS), signals indicative of the position, position quality, altitude, ground speed, ground track, date, time and status from a global navigation positioning system (GNPS) or a global positioning system (GPS) (hereinafter collectively referenced as a GPS), signals indicative of the glideslope deviation, a localizer deviation and the selected runway coordinates from an instrument landing system (ILS) and/or a microwave landing system (MLS) as well as other signals from other avionic subsystems. Therefore, for a conventional ground proximity warning system to be fully functional, the aircraft must not only carry the ground proximity warning system, but must also have a number of other subsystems, such as a radio altimeter, an ADC, an FMS, an IRS, an INS or an AHRS, a GPS and an ILS or a MLS. As will be apparent, each of these subsystems is quite expensive. However, most large commercial aircraft are mandated to have most, if not all, of these subsystems, such that the input parameters required by a conventional ground proximity warning system are readily available.
In contrast to commercial aircraft, general aviation aircraft, such as light turbine and piston aircraft, are not required to have many of the foregoing subsystems and, as a result, do not carry most of the foregoing subsystems since each subsystem is quite expensive. For example, most general aviation aircraft do not include a radio altimeter, an ADC, an INS or an IRS. Even though GPS is becoming increasingly more affordable and many general aviation aircraft therefore carry GPS equipment, conventional ground proximity warning systems cannot function properly based upon the parameters provided solely by the GPS without input from a variety of other subsystems that are not generally carried by general aviation aircraft. As such, a ground proximity warning system has been developed by Honeywell, Inc. that is specifically designed to operate based upon a reduced set of input parameters as described by U.S. patent application Ser. No. 09/534,222 entitled Ground Proximity Warning System and Method Having a Reduced Set of Input Parameters filed Mar. 24, 2000.
With respect to vertical speed, most general aviation aircraft do not carry the avionics subsystems that typically provide signals indicative of the vertical speed of the aircraft. In this regard, most general aviation aircraft do not carry an ADC, an INS or an IRS that typically provide signals indicative of the vertical speed for commercial aircraft. However, general aviation aircraft do have several options for obtaining a vertical speed value. For example, for general aviation aircraft that carry a GPS unit, the GPS unit may provide signals indicative of the vertical velocity of the aircraft. In addition, the GPS unit will provide signals indicative of the altitude of the aircraft from which the vertical speed of the aircraft can be determined by calculating the rate of change of the altitude of the aircraft. While the value of vertical velocity obtained from a GPS unit, either directly or by derivation from the altitude values, has relatively good resolution, such as 1 foot, the vertical velocity obtained from a GPS unit is subject to drift such that over the long term the vertical velocity obtained from a GPS unit is less reliable than generally desired.
General aviation aircraft also include means for determining the pressure altitude from which the vertical speed can be calculated based upon the rate of change of the pressure altitude. As described in U.S. patent application Ser. No. 09/255,670 entitled xe2x80x9cMethod and Apparatus for Determining Altitudexe2x80x9d filed Feb. 23, 1999, however, pressure altitude is subject to some errors based upon the calculation of pressure altitude from the actual outside air pressure, i.e., a local pressure measurement, as well as assumed internationally agreed standard atmosphere (ISA) values for pressure at sea level, temperature at sea level and temperature lapse rate, i.e., the assumed variation of temperature as a function of altitude. See Introduction to Flight, 3rd Edition (McGraw-Hill Series in Aeronautical and Aerospace Engineering), p. 79 (Nov. 1988). For example, most general aviation aircraft include an altitude encoder for measuring the pressure altitude, albeit only to a resolution of 100 feet. While some blind encoders offer better resolution than altitude encoders, blind encoders are still limited to a resolution of about 10 feet. As such, while general aviation aircraft do include means for determining the pressure altitude from which the vertical speed can be calculated, the estimation of vertical speed derived from the rate of change of the pressure altitude will have an undesirably poor resolution.
General aviation aircraft typically fly at much lower altitudes and in much closer proximity to the underlying terrain and other obstacles than commercial aircraft and would therefore appear to have at least as great, if not greater, of a need for an accurate measure of the vertical speed of the aircraft for use in a ground proximity warning system and the like. However, general aviation aircraft cannot generally generate an accurate representation of the vertical speed of the aircraft since general aviation aircraft do not carry the other subsystems, such as an INS, an IRS and an ADC, that are utilized by commercial aircraft to measure vertical speed.
A method, an apparatus and a computer program product are provided according to the present invention for accurately determining the vertical speed of an aircraft in a manner independent of signals provided by an air data computer, an inertial reference system and an inertial navigation system. The method, apparatus and computer program product of the present invention are therefore particularly well suited for general aviation aircraft that do not include some of the more expensive avionics subsystems, but that require an accurate estimation of the vertical speed of the aircraft for use in a ground proximity warning system and the like.
According to the present invention, a first vertical velocity of the aircraft is determined based upon a pressure altitude value associated with the aircraft. Typically, the first vertical velocity of the, aircraft is determined by the rate of change of a pressure altitude value over time. The pressure altitude can be measured by a variety of instruments, including an altitude encoder, a blind encoder and tap like. According to the present invention, a second vertical velocity of the aircraft is also obtained from a GPS receiver carried by the aircraft. In one embodiment, the second vertical velocity of the aircraft is obtained by receiving a series of altitude values from the GPS receiver over time and then determining the rate of change of the altitude values provided by the GPS receiver. In order to further improve the accuracy of the second vertical velocity, the rate of change of the altitude values provided by the GPS receiver can also be low pass filtered. Alternatively, the second vertical velocity of the aircraft can be obtained directly from a GPS receiver that is designed to measure the vertical velocity of the aircraft.
According to the present invention, the first and second vertical velocities are combined to determine the vertical speed of the aircraft. In this regard, the first and second vertical velocities are combined in such a manner to compensate for potential errors in the first and second vertical velocities. As such, the resulting vertical speed of the aircraft is more accurate than either of the first and second vertical velocities taken individually. In this regard, the first and second vertical velocities are combined by complimentarily filtering the first and second vertical velocities. More particularly, the first vertical velocity is preferably low pass filtered to remove high frequency noise that is attributable to the relatively low resolution of the first vertical velocity value. Additionally, the second vertical velocity is preferably high pass filtered to reduce errors due to long-term drift. Thus, the vertical speed of the aircraft is determined according to the present invention so as to have the best attributes of both the first and second vertical velocity values while eliminating the most common types of errors included within the first and second vertical velocity values. Thus, the present invention should provide an accurate estimation of the vertical speed of the aircraft, even though the vertical speed is not measured by an ADC, an INS or an IRS.
In order to further improve the accuracy with which the vertical speed of the aircraft is determined, at least one of the first and second vertical velocities can be weighted based upon its respective resolution. For example, the first vertical velocity can be weighted based upon the resolution of the pressure altitude value from which the first vertical velocity is derived. Thus, a first vertical velocity derived from pressure altitude values provided by an altitude encoder can be discounted to a greater degree than a first vertical velocity that is based upon pressure altitude values measured by a blind encoder since the blind encoder has a greater resolution than an altitude encoder.
According to one embodiment, the validity of the signals provided by the GPS receiver, including the signals from which the second vertical velocity of the aircraft is obtained, is monitored to insure that the vertical speed of the aircraft is only based upon valid data. In this regard, it is determined if at least a predetermined number of satellites are in view of the GPS receiver. If less than the predetermined number of satellites are in view of the GPS receiver, the second vertical velocity is maintained equal to its prior value, such as the last value of the second vertical velocity that was obtained based upon at least the predetermined number of satellites being in view of the GPS receiver. If legs than the predetermined number of satellites are in view of the GPS receiver for a continuous period that is at least as great as a predetermined time, a signal can be provided to indicate that the vertical speed is unreliable since the last valid value of the second vertical velocity may no longer be representative of the vertical speed of the aircraft.
According to one aspect of the present invention, the vertical speed of the aircraft is determined by an apparatus that includes a processor that determines the first vertical velocity of the aircraft based upon pressure altitude values, obtains a second vertical velocity from a GPS receiver and is adapted to combine the first and second vertical velocities to determine the vertical speed of the aircraft, such as by complimentarily filtering the first and second vertical velocities. According to another aspect of the present invention, the vertical speed of the aircraft is determined by a computer program product that includes a computer readable storage medium having computer readable program code means embodied therein. The computer readable program code means includes first computer instruction means for determining the first vertical velocity of the aircraft based upon pressure altitude values, second computer instruction means for obtaining a second vertical velocity of the aircraft from a GPS receiver and a third computer instruction means for combining the first and second vertical velocities to determine the vertical speed of the aircraft.
By combining the vertical velocity of the aircraft that is premised upon pressure altitude and obtaining the vertical velocity of the aircraft from the GPS receiver, an accurate estimation of the vertical speed of the aircraft can be obtained since the first and second vertical velocities are combined in such a manner that those errors inherent in the first and second vertical velocities are attenuated, thereby obtaining a measure of the vertical speed that is more accurate than either the first or second vertical velocity value is individually. By determining the vertical speed of the aircraft based upon pressure altitude signals and signals provided by a GPS receiver, however, the vertical speed of the aircraft can be determined in a manner independent of signals provided by a an ADC, an IRS or an INS. Thus, the method, apparatus and computer program product of the present invention are particularly well suited for general aviation aircraft that may include a GPS receiver, but that typically do not include more expensive subsystems, such an ADC, an IRS or an INS.